This invention relates to streaming cartridge tape drives, and more particularly to a streaming cartridge tape drive incorporating a current control circuit for delivering a constant and a symmetric write current to the magnetic write heads in the drive.
Streaming cartridge tape drives provide mass storage of magnetic information by writing long streams of serial data in a plurality of parallel streams on magnetic tape. The magnetic tape includes many such parallel data streams, each stream being written at a different vertical position on the tape. When the tape drive completes writing a serial data stream along the entire length of the tape at one vertical position on the tape, the tape drive reverses the direction of the tape and writes another serial data stream at a second vertical position on the tape. This back and forth method of recording data in a plurality of parallel streams on magnetic tape is called serpentine recording, and is well known in the streaming cartridge tape drive art.
Serpentine recording is advantageous in that it maximizes the amount of magnetic tape utilized by eliminating tape starts and stops inherent in tape drives which simultaneously record a plurality of data tracks along the entire width of the tape. To achieve high data transfer rates, streaming cartridge tape drives have relatively high speed transport systems which transport the magnetic tape past the magnetic heads at rates of speed of up to 90 inches per second.
Magnetic information is recorded on the tape as a series of magnetic poles, the transition between each differing magnetic pole representing a bit of information. The magnetic poles are produced by providing an alternating write current to the magnetic write head. The write current is first provided in one direction in the winding of the magnetic head to produce a magnetic pole and then in the opposite direction in the winding to produce an opposite magnetic pole on the tape. Because the magnitude of the write current affects the strength of the magnetic poles on the tape, it is highly desirable to ensure that the write current from one tape drive to another tape drive is substantially constant in order to enhance data integrity. It is also very desirable that within each tape drive, the write current provided in one direction in the magnetic write head winding is substantially equal in magnitude to the write current provided in the opposite direction in the winding. Write currents which are substantially equal in both directions in the write head winding are considered to be symmetric. When write current is symmetric and also constant from one tape drive to another, the accuracy of the retrieval of the magnetic information written to magnetic tape is maximized.
Streaming cartridge tape drives typically incorporate a pair of transistors connected to the winding of a magnetic write head in order to provide the write current to the head. The magnitude of the write current provided by these transistors depends on a number of factors which include the electrical tolerances of the associated biasing elements, such as resistors and power supplies, as well as the electrical characteristics of the transistors themselves. For example, each transistor has a baseemitter voltage V.sub.be which is dependent upon the electrical characteristics of the transistor. The value of V.sub.be varies from transistor to transistor and varies within a transistor as the temperature changes. These variances produce nonuniformities in the magnitude of the write current supplied to the magnetic write head. Since different transistors are used to provide current in different directions in the windings of the write head, the current supplied to the write head is asymmetric; that is, the magnitude of the current through the winding of one direction is not equal to the magnitude of the current in the winding in the opposite direction.
In addition, since the transistors and associated biasing elements in one tape drive may have different electrical characteristics than the transistors and biasing elements in another tape drive, the write current produced from tape drive to tape drive may vary. This variance in the magnitude of write current from drive to drive, as well as its asymmetry, adversely affect the accuracy of subsequent information retrieval from the tape.